function loadRss() {
  var a = new Ajax.Request( 
    rssUrl, 
    { 
      "method": "get", 
      "parameters": "feedurl=" + feedUrl, 
      onSuccess: renderRss, 
      onFailure: function(request) { 
        alert('読み込みに失敗しました'); 
      }
    } 
  ); 
}
function renderRss(request) {
  $('rss').innerHTML = request.responseText;
}